Git push Heroku master 报下边的错!

vagrant@homestead:~/Code/Laravel$ git push heroku master
.
.
remote:        > @php artisan package:discover
remote:        Discovered Package: fideloper/proxy
remote:        Discovered Package: laravel/tinker
remote:        Discovered Package: nesbot/carbon
remote:        Package manifest generated successfully.
remote: -----> Preparing runtime environment...
remote: -----> Checking for additional extensions to install...
remote: -----> Discovering process types
remote:        Procfile declares types -> web
remote:
remote: -----> Compressing...
remote:        Done: 17.7M
remote: -----> Launching...
remote:        Released v4
remote:        https://jia-laravel.herokuapp.com/ deployed to Heroku
remote:
remote: Verifying deploy... done.
To https://git.heroku.com/jia-laravel.git
 * [new branch]      master -> master
error: update_ref failed for ref 'refs/remotes/heroku/master': cannot lock ref 'refs/remotes/heroku/master': unable to create directory for .git/refs/remotes/heroku/master